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So we're going to talk about the concept of fitness and how it relates to natural selection. But first, let's go over what natural selection is. So if you have a member of a population that has a special genetic trait, like being really strong, then that person is more likely to live to an age where he or she can reproduce and pass on that special trait to offspring. Now, this example only really deals with surviving until the age where reproduction is possible and doesn't really deal with the person's ability to actually reproduce once it gets to that age. Now, you should also remember that populations as a whole will evolve by natural selection, and not individual members of a population. So what is fitness? And what does this term mean? Well, fitness refers to an organism's total ability to pass on traits to offspring. And we can look at fitness as a combination of an organism's ability to survive to an age where it can reproduce, as well as its ability to actually reproduce once it gets there. So our strong guy probably has a higher survival rate than the average person, but actually reproduces the same as everyone else. And how well someone can reproduce is determined by that person's fecundity, which is what we're going to talk about today. So what's fecundity, exactly? Well, fecundity refers to how easily and how often an organism can produce offspring. And when you look at asexual reproduction instead of sexual reproduction, fecundity of bacteria like E. coli is determined by how quickly the E. coli cells can divide and increase their population size. When looking at sexual reproduction, fecundity refers to how well an organism can mate with another, and in the case of mammals like humans, carry and ultimately birth offspring. Now, when looking at the fecundity of humans, people often think that it only applies to female fertility since the females are the ones that are actually carrying the children. But fecundity can also be a measure of a male's ability to produce offspring with a female mate. Now, fecundity is directly related to natural selection because, like any other special trait, high fecundity is selected for. So let's look at an example. Let's say we have a population of six people, two of which are red and four of which are blue. Now, the red and the blue people have the same chance of surviving to an age where they can reproduce, which is 50%. And what this means is that one of our red people will survive to an age where they can reproduce and two of our blue people will survive. But the difference between the two is that the red people who survive will each be able to produce four offspring, whereas the blue people who survive will only be able to produce three each. So by our second generation, we have four red people and six blue people. Now, two of our red people, 50%, will survive. And three of the blue people, also 50%, will survive as well. But once again, the surviving red people will each have four offspring, while the surviving blue people will only have three offspring each. So this leaves our third generation with eight red people and nine blue people. If we now number our generations generations 1, 2, and 3, we see that in generation 1, 33% of the population was red, while 67% of the population was blue. In generation 2, 40% were red and 60% were blue. And by generation 3, 47% were red and 53% were blue. And this increases all because the red people had a special trait of higher fecundity, which made them more able to have offspring than the blue people. And this means that fecundity is selected for by natural selection the same way a trait that benefits survival like strength would be. Now, another interesting way that fecundity is selected for has to do with mate selection. Now, when looking for a female mate, many males associate an attractive woman with words like "curvy." And a curvy female would be one that has a healthy and robust body, that was fit for bearing children. And this ability to easily bear children is a direct indicator of high fecundity. So what did we learn? Well, first we learned that natural selection will select for individuals with traits that give them high fitness. And we can divide fitness into traits that will help with survival and traits that will help with reproduction, which are those that increase fecundity. Second, we learned that fecundity is selected for by natural selection, just like any other special trait would be.